Pipe Wall Thickness Calculation

Additional loads are best carried by a casing. In the US, NG pipelines go under railroad tracks in casings 6 inches or so in diameter more than the actual line, and the line pipe itself was in a heavier wall through the casing. Existing underground piping can have the loads transferred to adjoining piles. A bridge of this sort permitted siting a crane directly over a 9 foot sewer line.

Thank you for your answer, can you help me in my first question as well?

Regards,

Hi Farzad, with respect to your first question and according I know, for the wall thickness pipe calculation, first you have to know the design conditions such as Temperature, Pressure, and coorosion allowance, with this, you can use the ASME31.3 to get the thickness, once you have this value, you have to choice the schedule of pipe to use from a comercial table of NPS (Nominal Pipe Size). By other hand and with respect to the Rating of the flage to be used, you have to calculate the MAWP; Maximum Allowable Working Pressure, with the design conditions, and once gotten this value, you have to go to ASME16.5 and you have choice the rating that you have to use, after you can specify the flange including the schedule which is the thickness of the neck of the flange. I hope this information being useful for your first question and excuse me if I have some mistake during the redaction of this note, but english is not my mother language.

Hello Arrow:

Thanks for your response,

For example we have a 600# rating class,

Up to 10" we are following the schedule (Pipe wall thickness) in accordance with B31.3 formula, and the design pressure also is in accordance with ASME B16.5 which is 1290 PSIG for up to 200 deg F.

My question is created here:

For size 12" and above our Design pressure is less than 600# and also more than 300# (e.g for up to 200 deg F design pressure is 1100 PSIG), our piping class's rating is still 600# but we are not going to consider 1290 PSIG design pressure we put 1100PSIG as design pressure in pipe wall thickness calculation formula (ASME B31.3).

(for having less pipe thickness in comparison with ASME B16.5 for economy reasons)

I want to know is it correct or are we allowed to do that?

Thanks

Hi Farzad, if I am understanding well, you mean that you want reduce costs reducing the design pressure in the design, all this is right whether you are sure of the value to be used as design pressure, and that this value is going to be lesser than operating pressure. The ussual way is having all the design conditions already defined before making thickness calculation, and generally these values are gotten from process department or from a P&ID; Process and Instrument Diagram.

For your first question, I would think that you could design like that as long as you have documentation proving that every component in your system can withstand the pressure. Now that's just me speaking so you might want to read some of B31.3 and B16.5 before you do anything.

However, most companies make the flange the weakest component so they can control where the failure will occur. Read this CR4 thread for some details...

http://cr4.globalspec.com/thread/52302/Why-are-Flanges-the-Weakest-Part-of-a-Piping-System

For your second question, you want to handle additional loads by adding the stresses together. You really need a pipe analysis done if you combine loads like this. To my knowledge, B31.3 doesn't give calculations for additional loads.

One way that I've done it is to add the thickness required for pressure to that required for something else (like bending). This is being a little extra conservative but it works.

Thanks cingold and others for your posting

To explain more:

E.g. we have a piping class with rating 600# in a Piping Material specification as follow:

According to this Pipe class

Pressure is: 1100 psig

Temperature is: 180 deg F

Although the Design pressure is less than rating 600# (which is max 1500 psig for the related temperature according to ASME B16.5) it is also much more than rating 300# (which is max 750) the best rating for this class is 600#.

For up to 10" we have considered the design pressure of 1500 psig for our pipe wall thickness calculation. (According to B31.3 formula)

After this size due to economic reasons we want to consider Design pressure 1100psig which is our actual design pressure (according to the mentioned information) for our Pipe wall thickness calculation.

My question is,

Is it correct to do that? Is it a normal practice? If it is not what is the good engineering practice?

Sometimes in pipe classes we have a schedule and wall thicknesses for sizes up to 10" or 12" but after that we have a note which says

To be calculated, I want to know what it means.

I think it means that they will be calculated according to real design pressure (like the above example), am I right?

Thanks

I'm trying not to be too specific because there are so many variables that go into designing pipe specs vs. one line that I would hate to give you the wrong answer.

Typically, you want to design to flange limit. Your design pressure is 1100 psig now but what happens if you need to increase that in the future? Have you looked at the cost of designing to flange limit vs. your design pressure?

I think I said this before though, as long as you have engineering calculations to back up your design then you should be ok. If you do the calculations on each component through each individual standard (like B31.3) then you should be fine.

Usually, pipe specs are made to make pipe selection easy. It's much easier to copy parts off of a piece of paper than to calculate each one.

The rule we use at our plant is you are supposed to follow the pipe specs. If you want to deviate from the pipe specs like you are doing then you fill out a deviation request and prove that what you are doing is safe.

Hi Farzad, cingold is right, in few words and summarizing, if your project or your design has to meet with piping specifications such as a piping class, you shall use the rating that it has been indicated in the piping class, if is not so, and such as has indicated cingolg, you can do that you have thought.

Thanks cingold and others for your posting

To explain more:

E.g. we have a piping class with rating 600# in a Piping Material specification as follow:

According to this Pipe class

Pressure is: 1100 psig

Temperature is: 180 deg F

Although the Design pressure is less than rating 600# (which is max 1500 psig for the related temperature according to ASME B16.5) it is also much more than rating 300# (which is max 750) the best rating for this class is 600#.

For up to 10" we have considered the design pressure of 1500 psig for our pipe wall thickness calculation. (According to B31.3 formula)

After this size due to economic reasons we want to consider Design pressure 1100psig which is our actual design pressure (according to the mentioned information) for our Pipe wall thickness calculation.

My question is,

Is it correct to do that? Is it a normal practice? If it is not what is the good engineering practice?

Sometimes in pipe classes we have a schedule and wall thicknesses for sizes up to 10" or 12" but after that we have a note which says

To be calculated, I want to know what it means.

I think it means that they will be calculated according to real design pressure (like the above example), am I right?

Thanks

Thanks cingold and others for your posting

To explain more:

E.g. we have a piping class with rating 600# in a Piping Material specification as follow:

According to this Pipe class

Pressure is: 1100 psig

Temperature is: 180 deg F

Although the Design pressure is less than rating 600# (which is max 1500 psig for the related temperature according to ASME B16.5) it is also much more than rating 300# (which is max 750) the best rating for this class is 600#.

For up to 10" we have considered the design pressure of 1500 psig for our pipe wall thickness calculation. (According to B31.3 formula)

After this size due to economic reasons we want to consider Design pressure 1100psig which is our actual design pressure (according to the mentioned information) for our Pipe wall thickness calculation.

My question is,

Is it correct to do that? Is it a normal practice? If it is not what is the good engineering practice?

Sometimes in pipe classes we have a schedule and wall thicknesses for sizes up to 10" or 12" but after that we have a note which says

To be calculated, I want to know what it means.

I think it means that they will be calculated according to real design pressure (like the above example), am I right?

Thanks